The origins of the Amazon can be traced back to WW1, which highlighted a need for a military lorry combining on and off-road capabilities. The off-road performance requirements could be satisfied with a relatively simple transmission, in the form of a rigid six-wheeler with two driven rear axles and a single, undriven, front axle.
This 1933 Amazon RSW looks rather overloaded.
Thornycroft based its design for such a vehicle on existing components. Thus, in early 1926, the A1 RSW was introduced, based on the four-wheel 1.5 ton (1,524kg) A1 lorry introduced in 1924. All major units of the A1 RSW came from the A1, including the 40bhp four-cylinder FB4 petrol engine, with the addition of a two-speed auxiliary gearbox, giving direct drive and a low ratio for off-road use, which, with the normal four-speed gearbox, gave eight forward and two reverse speeds. Power was transmitted by a propeller shaft to the worm gear of the first rear axle’s differential. A short, stiff shaft transmitted power from the first rear axle to the worm of the second rear axle.
Patented by Thornycroft, the A1 RSW’s rear suspension complied with War Office requirements for off-road operation, by ensuring equal weight distribution on all four (rear) driving wheels, and freedom from spring distortion under all conditions of driving and braking and irrespective of the relative movement of the driving axle.
The A1 RSW could carry 2.5 tons (2,540kg) and 1.5 tons (1,524kg) on and off-road, respectively. In addition to its military application, this fairly simple, conventional vehicle was seen to have potential in undeveloped countries where roads might be poor or non-existent.
The A1 RSW was developed into the very similar A3 RSW which entered production in 1926, initially with the same on and off-road carrying capacities as its predecessor, but upgraded for 1927 to corresponding capacities of 3 tons (3,048kg) and 2 tons (2,032kg), respectively. However, a rigid six-wheeler with on and off-road capability of larger capacity than the A3 RSW was needed, and, in 1927, Thornycroft introduced the Type XB capable of carrying 5 tons (5,080kg) on-road and 3 tons (3.048kg) over rough country.
An articulated outfit headed by an Amazon RSW.
The XB continued with the A3 RSW’s formula, by 1) complying with War Office requirements for RSWs operating off-road, mentioned earlier, 2) using existing components, 3) using both rear axles as driven units and 4) incorporating an additional two-speed auxiliary gearbox with a low ratio for off-road use. The rear suspension allowed a difference of 9ins (22.9cm) in rear axle levels, either axle being able to tilt to an angle of 13 degrees, and each was cushioned on a rubber block at the top or bottom of the tilt. The vehicle was powered by the, then, newly-introduced MB4 four-cylinder petrol engine giving 60bhp@1,800rpm.
A long-wheelbase version of the XB was offered in addition to the standard model, although the load capacity of both normal and long-wheelbase models was the same. In 1931, the on-road load capacity was increased from 5 tons (5,080kg) to 6 tons (6,096kg). Also that year, when Thornycroft started naming its vehicles, the XB became known as the Amazon, while retaining the XB designation. Produced in parallel with the XB was the EC RSW with an on-road carrying capacity of 5 tons (5,080kg), introduced in late 1929 and powered by a WB6 six-cylinder side-valve petrol engine giving 78bhp@1,500rpm. The EC was available with long and normal wheelbases, and became the six-cylinder version of the Amazon, becoming known in 1932 as the Amazon XB/WB6 with an increased 6 ton (6,096kg) on-road load capacity. Thus, Amazons available in 1932 comprised the four-cylinder XB/MB4 and the six-cylinder XB/WB6, each in normal and long-wheelbase versions, the MB4 and WB6 notation indicating the engine type.
A forward-control Amazon RSW rolling chassis.
The Amazon range was extended in 1933 to include a diesel version, the XB/CIND4, fitted with Thornycroft’s four-cylinder CIND4 diesel giving 82bhp@1,800rpm. The six-cylinder petrol version of the Amazon received the powerful new AC6 engine with overhead valves giving 99bhp@2,100rom, no less than 25 per cent more than the side-valve ‘six’ that it replaced. The diesel (CIND4) and both petrol versions (MB4 and AC6) were offered in long and normal wheelbase forms, all with 6 ton (6,096kg) on-road load capacities and less off-road.
Thornycroft decided to drop the seemingly-underpowered 60bhp four-cylinder petrol version and, in late 1934, the Amazon was offered in two basic versions, the six-cylinder 99bhp petrol XB/AC6 and four-cylinder 82bhp diesel XB/CIND4, each in normal and long-wheelbase form. However, by mid-1935, the Amazon RSW was no longer listed for the British market, although it was available as an articulated 12 tonner (12,192kg) with a six-wheel tractive unit, with the same designation as the Amazon RSW, namely XB/CIND4 and XB/AC6. The AC6 engine in the latter was uprated to 104bhp, from the 99bhp output of the Amazon RSW. Even the articulated Amazon was no longer listed in the UK in 1936, but it, and the Amazon RSW, received export orders.
Three examples of Amazon export orders are given below. The front axles of all these Amazons were positioned behind the engines, giving the ‘snout’ type appearance which characterised, for instance, the earlier Taurus lorry
No Thornycroft RSWs of any types were listed for UK sale in 1937 and 1938, and only one was available in 1939, the new Dreadnought 10 tonner (10,160kg) for on-road use only, with two front axles and one rear axle.
Information for these pages comes from contemporary Thornycroft technical and other data, as well as contemporary copies of The Commercial Motor.
Amazon “XB/AC6 Long” rigid six-wheeled freight chassis
To carry a net load of 6 tons (6,096kg) over good roads, and reduced loads over rough tracks or cross-country, with a body allowance not exceeding 20cwt (1,016kg)
Combined with the latest developments in design and construction, ease of maintenance and accessibility have been given particular attention throughout.
Six-cylinder 99bhp at 2,100rpm, type AC6, watercooled petrol engine, 4.375ins (111mm) bore, 5.25ins (133.4mm) stroke. Cubic capacity 7,749cc. Three point suspension of special construction, stresses in crankcase due to frame distortion are thus entirely eliminated.
Monobloc construction assisting rigidity of engine and fitted with special wet cast iron liners. The cylinder head is easily detachable for decarbonising and grinding-in the valves.
Overhead valves operated by push rods and rockers from single camshaft housed in the crankcase, and driven from the front of the engine by a triplex chain with an adjusting device, rendering the drive extremely reliable. The tappets are easily removed through three large covers on the side of the cylinder block and access to the rockers is obtained by removal of an aluminium cover secured by hand screws.
A high-grade casting of special alloy, designed to give extreme rigidity with minimum weight. The cylinder block, crankcase and main bearing caps, reinforced with steel keeper plates, are secured together by through-bolts and stiffening pads at the points of maximum stress. The bottom half is easily detachable for the inspection and taking-up of the main and big end bearings.
A dynamically and statically balanced nick
el-chrome steel crankshaft of robust design, machined all over and carried in seven white-metal lined bearings of 2.875ins (73.03mm) diameter. A very efficient vibration damper is fitted to maintain smooth running over the whole speed range.
Force-feed lubrication, special attention being paid to the filtering of the oil. From an efficient external oil filter, which can be removed without loss of oil or the necessity to drain the sump, the oil passes to the main and connecting rod bearings, also to the camshaft bearings and camshaft thrust bearing. Oil is fed to the overhead valve gear, under reduced pressure, through the hollow rocker shaft. Provision is made for cooling the oil by ribs cast on the filter and on the sump, the former being placed in the air stream from the fan. The sump, which has a capacity of 3½ gallons (15.9 litres), is easily drained by unscrewing a single nut on the side of the engine and without the necessity of crawling underneath the chassis.
By high tension, automatic-advance magneto. The magneto is located at the front of the engine.
Float-feed automatic type with special starting jet in addition to a pilot jet for slow running. Induction manifold heated by “hot spot” from exhaust. The throttle is controlled by accelerator pedal and an adjustable lever on the steering column. A specially designed governor is driven from the camshaft.
By pump, direct driven from the engine, and fan driven from a pulley on the dynamo shaft. The pump is of the impeller type and when out of action will not impede thermo-syphon cooling.
A vertical gilled-tube radiator of distinctive appearance, with detachable top and bottom water vessels, mounted on rubber buffers.
A single-plate dry clutch of large diameter and area, with specially-prepared fabric surfaces. The lightness of the driven member and the efficiency of the clutch stop make gear-changing simple and rapid.
Of unit construction, providing four forward speeds and reverse, top gear being direct drive. The gear shafts, which are 2½ins (63.5mm) diameter, run in large-size roller bearings, and the gear wheels, with teeth 1.3125ins (33.34mm) wide, are of special contour and ground to obtain silent running. A speedometer drive is incorporated with the gearbox.
The reduction ratios are as follows
Bolted to the rear face of the main gearbox and operated by a separate lever, providing a further reduction of 2.5 to 1 which affords an additional range of exceptionally low gear ratios for extremely bad conditions. The width of the gear wheels is 1.625ins (41.28mm).
Change speed control
Change-speed lever mounted direct on gearbox, light in operation, permitting easy engagement of gears.
The drive is transmitted from the auxiliary gearbox by hollow propeller shafts with enclosed metallic universal couplings. An intermediate propeller tube is interposed between the auxiliary gearbox and the propeller shaft to the forward driving axle, the rear end being supported by a ball bearing flexibly attached to one of the frame crossmembers.
Two nickel steel casings with full-floating differential shafts of 2ins (50.8mm) diameter, driven by overhead worms with a reduction ratio of 7¼ to 1. For exceptionally difficult conditions of service an alternative reduction ratio of 8¼ to 1 can be provided, without extra charge, if specified when the chassis is ordered.
Front axle 3ins x 3¼ins (76.2mm x 82.3mm) section axle body of medium carbon steel forged in one piece. The 2.1875ins (55.56mm) diameter swivel arms are of nickel steel with taper-roller thrust bearings on the pivot pins, taking the weight and giving easy steering. The track rod is adjustable.
The foot brake, assisted by a powerful vacuum-servo cylinder, operates internal expanding shoes in 20ins (50.8cm) drums on the wheels of the rearmost driving axle, and the hand brake acts on the 20ins (50.8cm) diameter drums of the foremost driving axle. Both these brakes, however, take effect on all four wheels of the bogie, by virtue of the short propeller shaft between the axles. All the brake gear facings consist of large fabric pads ensuring long life with a minimum of adjustment.
Worm and wheel type with taper-roller bearings. The worm wheel is complete, thus permitting a fresh sector of the wheel to be brought into service should backlash develop through wear.
Lubrication of chassis
By grease-gun and oil-gun, the nipples being grouped in accessible positions.
A pressed steel frame of deep channel section, reinforced in the centre. Channel-type crossmembers with wide gussets correctly placed give an extremely rigid frame construction without employing excessive weight of material.
A 29 gallon (131.8 litres) steel tank with petrol gauge is carried on the dash where it is easily accessible for filling.
Four inverted semi-elliptic springs independently pivoted at their centres to brackets rigidly attached to the chassis frame, designed under Thornycroft Patent No. 259,322, to give even distribution of weight on all four wheels of the bogie under all conditions, irrespective of the relative movements of the axles, thus obtaining equal driving results from each wheel.
The springs form the only connection between the axles and the frame, no torque tubes or radius rods being employed, the outstanding feature of this design being that the torque reaction is transmitted to the frame through the road springs, which ensure a cushioning effect, and thus relieve the frame of shocks.
The suspension is designed to permit a 9ins (22.9cm) difference in driving axle levels and also to allow either axle to tilt to an angle of 13 degrees (when one wheel will be approximately 18ins (45.7cm) above the other) without causing any distortion of the springs. Rubber buffers fixed above and below the axles prevent excessive movement.
Made of silico-manganese steel, ensuring easy riding. The front springs, which are designed to be approximately flat under normal load, are secured to the axle by our patented type of relieving plate (Patent No. 199,522), which enables the holding-down bolts effectively to withstand the stresses due to flexing of the springs without suffering undue elongation.
In order to relive the central bolt of shear stress which would otherwise be caused by horizontal forces due to road shocks, etc., the holding-down bolts are inclined to one another, and special ears are provided on the eye-plate to grip them.
Wheels and tyres
10.50ins-20ins (26.7cm-50.8cm) low pressure tyres are fitted to disc wheels, single on all axles. Alternatively, 9.75ins-20ins (24.8cm-50.8cm) low pressure tyres, single on all axles, may be fitted at an extra charge. All wheels are detachable and interchangeable.
Exceeding 5 tons (5,080kg) weight unladen. £48.
Standard chassis weight
With 10.50ins-20ins (26.7cm-50.8cm) tyres, battery, standard lamps and dynamo, minimum oil in engine, gearbox and driving axles, but excluding petrol, water, tools, spare wheel and carrier: 4 tons 14cwt 2qrs (10,584lbs/4,801kg)
Standard chassis equipment
A 12 volt electric lighting set with dip and switch headlamps, two wing lamps, combined tail and “stop” light, 105 amp-hour battery, electric horn operated from centre of steering wheel, electric starter, engine speed governor, speedometer and mileage recorder, spare wheel with tyre and carrier, mechanical tyre pump, and a comprehensive kit of tools.
With overseas rough track chassis the following spare parts are supplied:
The following may be supplied at extra charge.
Outline Diagram of Amazon“XB/AC6 Long” 120kb pdf